The present disclosure relates generally to improving directional control on a multi-engine aircraft, and more particularly to ensuring sufficient directional control during roll maneuvers for an aircraft experiencing engine thrust asymmetry.
In typical multi-engine aircraft configurations, the aircraft engines may be mounted symmetrically on opposing wing structures or on opposing sides of the aircraft fuselage. This symmetrical mounting creates equivalent moment arms for each engine with respect to the vertical axis of the aircraft, which negates any yawing moment induced by any particular engine when both engines are producing equivalent thrust. However, in the event of an engine failure, asymmetric throttle command or some other event that results in one engine producing greater thrust than the opposing engine, several adverse effects may take place.
The primary effect of asymmetric thrust is that the aircraft will tend to yaw in the direction of the engine producing lower thrust because of the greater torque generated about the vertical axis by the engine producing the greater thrust. This effect is often compounded in an engine failure situation where an inoperative engine may produce additional drag while the compressor fan blades create a windmilling effect in response to the incoming airflow. To overcome and control this induced yaw, a counteracting yawing moment may be introduced by deflecting the rudder. When the rudder is deflected, the corrective yawing moment produced by the rudder about the aircraft's vertical axis is dependent upon the velocity of airflow across the rudder, which in turn is dependent on the air speed. As the aircraft decelerates, the rudder will need to be deflected further to maintain yaw control.
A problem arises, however, when a speed is reached where the yawing moment produced by the fully deflected rudder will just balance the thrust moment. If a roll maneuver is performed at this condition in the direction to roll towards the operative engine, there is no additional rudder deflection available to prevent the buildup of aircraft sideslip angle. Rudder deflection is often required during roll maneuvers, particularly at lower airspeeds, to oppose adverse yawing moment during a roll maneuver and to generate the required body axis yaw rate. If the required rudder deflection for a given roll rate is not used, the result may be an undesirable buildup of sideslip angle. Excessive sideslip angle in a roll may prevent the airplane from rolling at the rate and to the angle that the pilot intended. The amount of adverse sideslip angle may be dependent on the roll rate of the roll maneuver. This may occur primarily during a relatively low-speed rolling maneuver in which the aircraft is rolling toward the operative engine.
For a given change in bank angle, a high roll rate will produce a larger sideslip angle than a lower roll rate. Under normal operating conditions the rudder can deflect during rolling maneuvers to control this sideslip due to roll rate. In asymmetric thrust conditions, where the rudder can be fully deflected to control the thrust asymmetry, rolling maneuvers cause large sideslip angles because the rudder cannot be deflected further.
For multi-engine airplanes, a maneuver that may demand the greatest amount of directional stability and directional control power is a rapid rolling maneuver with high asymmetric thrust towards the higher thrust engine. Greater use of flight control augmentation has reduced vertical tail size requirements for traditional sizing conditions of directional stability and directional trim. Reducing the vertical tail size can cause the asymmetric thrust roll maneuver to be an important design condition.
Existing solutions include increasing the vertical tail size, increasing operating speeds, using a constant roll rate limiter, using a constant thrust reduction, or using a thrust reduction that is a function of roll rate.
Increasing the vertical tail size increases directional stability and leaves more rudder deflection available to control high roll rates. Increasing the vertical tail size, however, adds weight and drag to the aircraft in all conditions whether the additional directional stability is needed or not; thereby increasing its operating costs and reducing the airplane's value.
Increasing operating speeds has the same effect on control power as increasing the tail size. Increasing operating speeds, however, increases required takeoff and landing distances, thus decreasing available airport/payload combinations; thereby reducing the airplane's value.
Using a constant roll rate limiter would be beneficial, but would reduce aircraft roll rate in all situations, including those where large roll rates are controllable and might even be required; for example, in collision avoidance maneuvers or a recovery from an upset.
Using a constant thrust reduction would reduce the rudder required to control a thrust asymmetry, making it available to control the sideslip due to roll rate. Using a constant thrust reduction, however, can increase required takeoff distances and/or limit payload, thus decreasing available airport/payload combinations; thereby reducing the airplane's value.
Using a thrust reduction that is a function of roll rate and sideslip angle works the same way as a constant thrust reduction, but is designed to only operate in situations where additional directional control is needed. Using a thrust reduction that is a function of roll rate does not have the same drawbacks as using a constant thrust reduction, but it does present increased complexity as it needs to interface with not only the flight control system but also the engine control system.
There is a need for a solution that is free of the drawbacks of existing solutions.